Molecular imaging has the potential to detect disease, disease progression or therapeutic effectiveness earlier than most conventional methods in the fields of oncology, neurology and cardiology. Of the several promising molecular imaging technologies having been developed such as optical imaging, magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), and positron emission tomography (PET), PET is also of particular interest for drug development because of its high sensitivity and ability to provide quantitative and kinetic data.
For example positron emitting isotopes include carbon, iodine, fluorine, nitrogen, and oxygen. These isotopes can replace their non-radioactive counterparts in target compounds to produce tracers that function biologically and are chemically identical to the original molecules for PET imaging, or can be attached to said counterparts to give close analogues of the respective parent effector molecule. Among these isotopes, 18F is the most convenient labelling isotope due to its relatively long half life (110 min) which permits the preparation of diagnostic tracers and subsequent study of biochemical processes. In addition, its low β+ energy (634 keV) is also advantageous.
The nucleophilic aromatic and aliphatic [18F]-fluoro-fluorination reaction is of great importance for [18F]-fluoro-labelled radiopharmaceuticals which are used as in vive imaging agents targeting and visualizing diseases, e.g. solid tumours or diseases of brain. A very important technical goal in using [18F]-fluoro-labelled radiopharmaceuticals is the quick preparation and administration of the radioactive compound.
Monoamine oxidases (MAO, EC, 1.4.3.4) represent a distinct class of amine oxidases. Monoamine oxidases are present in two isoforms known as MAO-A and MAO-B (Med. Res. Rev. 1984, 4: 323-358). Crystal structures of MAO-A and MAO-B complexed by ligands have been reported (J. Med. Chem. 2004, 47: 1767-1774 and Proc. Nat. Acad. Sci. USA 2005, 102: 12684-12689).
In the human brain the presence of MAO-B predominates over MAO-A. Cerebral MAO-B levels increase with age and are further up-regulated in the brains of Alzheimer's disease (AD) patients mostly due to an increase of reactive astrocytes. As astrocyte activity and, consequently, the activity of the MAO-B system is up-regulated in neuroinflammatory processes, radiolabelled MAO-B inhibitors may serve as an imaging biomarker in neuroinflammation and neurodegeneration, including Alzheimer's disease.
Inhibitors that are selective for either MAO-A or MAO-B have been identified and investigated (e.g. J. Med. Chem. 2004, 47: 1767-1774 and Proc. Nat. Acad. Sci. USA, 2005, 102: 12684-12689).
Deprenyl (compound A), a MAO-B inhibitor (Biochem. Pharmacol. 1972, 5: 393-408) and clorgyline (B), a MAO-A inhibitor (Acta Psychiatr. Scand. Suppl. 1995, 386: 8-13), are potent monoamine oxidase inhibitors inducing irreversible inhibition of the respective enzymes. The (R)-isomer of deprenyl (Selegilin®, compound (R)-A) is a more potent inhibitor than the (S)-isomer (not shown).

Neuroprotective and other pharmaceutical effects have also been described for inhibitors (Curr. Pharm. Des. 2010, 16: 2799-2817, Nature Reviews Neuroscience 2006, 295: 295-309; Br. J. Pharmacol. 2006, 147: 5287-5296, J. Alzheimers Dis. 2010, 21: 361-371, Prog. Neurobiol. 2010, 92: 330-344).
MAO-B inhibitors are for example used to increase DOPA levels in CNS (Progr. Drug Res. 1992, 38: 171-297) and they have been used in clinical trials for the treatment of Alzheimer's disease (AD) based on the fact that an increased level of MAO-B is involved in astrocytes associated with Alzheimer plaques (Neuroscience 1994, 62: 15-30).
Fluorinated MAO inhibitors have been synthesised and biochemically evaluated (Kirk et al., Fluorine and Health, A. Tressaud and G. Haufe (editors), Elsevier 2008, pp. 662-699). 18F and 11C labelled MAO inhibitors have been studied in vivo (Journal of the Neurological Science 2007, 255: 17-22; review: Methods 2002, 27: 263-277).
18F labelled deprenyl and deprenyl analogues (D) and (E) have also been reported (Int. J. Radiat. Appl. Instrument. Part A, Applied Radiation Isotopes, 1991, 42: 121; J. Med. Chem. 1990, 33: 2015-2019 and Nucl. Med. Biol. 1990, 26: 111-116, respectively).

Amongst said 11C labelled MAO inhibitors, [11C]-L-Deprenyl-D2, also referred to as DED ([11C]-L-bis-deuterium-deprenyl), has been widely used by multiple groups to study CNS diseases with regard to their impact on MAO-B activity, such as epilepsy (Acta Neurol. Scand. 2001, 103: 360; Acta Neurol. Scand. 1998, 98: 224; Epilepsia 1995, 36: 712), amyotrophic lateral sclerosis (ALS, see J. Neurolog. Sci. 2007, 255: 17), and traumatic brain injury (Clin. Positron Imaging 1999, 2: 71).

Moreover, a comparative multitracer study including DED has been performed in patients suffering from Alzheimer's disease (AD) and healthy controls (NeuroImage 2006, 33: 588).
DED has been furthermore used in studies on the effect of smoking and age on MAO-B activity (Neurobiol. Aging 1997, 18: 431; Nucl. Med. Biol. 2005, 32: 521; Proc. Nat. Acad. Sci. USA 2003, 20: 11600; Life Sci. 1998, 63: 2, PL19; J. Addict. Disease 1998, 17: 23).
The non-deuterated analogue of DED, [11C]-L-deprenyl, binds very rapidly and irreversibly to MAO-B. As a result, the tracer may be trapped at a rate similar to or higher than its delivery by plasma, rendering PET images of regions with high MAO-B levels and/or low blood flow representing perfusion rather than MAO-B activity. The binding of DED is slower due to a kinetic isotope effect and thus DED allows for a more accurate assessment of MAO-B activity as its non-deuterated counterpart (see e.g. J. Nucl. Med. 1995, 36: 1255; J. Neurochem. 1988, 51: 1524).
WO 2009/052970 A2 discloses novel 18F labelled analogues of L-Deprenyl. Compound F shown below features favourable uptake in baboon brain and improved properties, such as superior metabolic stability, as compared to [11C]-L-Deprenyl and the aformentioned 18F labelled MAO-B inhibitors D and E. It can be anticipated that deuteration at the propargylic position, as introduced in DED, will also result in a reduction of the trapping rate of compound F (Fowler et al. J. Neurochem. 1988; 51: 1524-1534).

For the sake of clarity, the reader is referred to the fact that the synthesis of F, alike the preparation of suitable precursors thereof such as J from alcoholic intermediates such as G is thought to proceed via a rearrangement reaction involving an aziridinium ion H. Said rearrangement may give rise to regiosiomeric mixtures of products as exemplified here. Thus, the regioisomeric precursors J1 and J2, due to the leaving group qualities of their chloro groups, can be equilibrated under suitable conditions, whilst F can be readily separated from its secondary regioisomer and is stable towards equilibration. For additional information on the aziridinium ion rearrangement see e.g. P. Gmeiner et al., J. Org. Chem. 1994, 59: 6766. The aziridinium rearrangement proceeds in a stereospecific manner, as described in WO 2010/121719 A1 (see also the aformentioned publication and J. Cossy et al., Chem. Eur. J. 2009, 15: 1064).
